The Anatomy of Chlamydoselachus 



341 



tary character of the cartilaginous rods, particularly the radials, that stiffen the basal 

 portions of the fins. These rods belong to the endoskeleton and will be further con' 

 sidered in their proper place. In all the fins there is a wide expanse supported only by 

 fine dermal fin rays. From the viewpoint of adaptation to environment, one may say 

 that softness and flexibility of the fins is an advantage to a fish that must make its way 

 through crevices in a rough sea bottom. In such a situation, stiff fins might be a decided 

 impediment. Evidently Chlamydoselachus is not a rapid swimmer, since it must depend 

 for locomotion partly upon serpentine movements of a slender body. 



Text-figure 5. 



Restoration of the Devonian shark, CladoseUche. Its iins were supported by simple 



parallel rods of cartilage extending nearly to the margin. 



After Dean, 1909, Fig. 41. 



In the earliest fossil remains of sharks that appear to have left modern descendants, 

 the parallel rods of cartilage (radials) that support each fin extend almost to its margin, 

 so that the entire fin must have been fairly rigid (e. g., as in Cladoselache, Text 'figure 5). 

 In living sharks there has been a reduction and modification of the radials and a correspond' 

 if^gly greater dependence on dermal rays for stiffening the fins. In Chlamydoselachus 

 the reduction of the radials has proceeded to an unusual degree but without a compen' 

 sating development of the dermal rays. 



The shortness and breadth of base of the fins of Chlamydoselachus bring to mind the 

 fin'fold theory (Thacher, 1877; Balfour, 1878; Mivart, 1879) for the origin of the fins of 

 fishes; but fins that are broad and short are found in some of the most highly specialized 

 sharks and more notably in the skates and rays. So this form of fin is not necessarily 

 primitive. In Chlamydoselachus, the shortness and breadth of the fins are in strict harmc 

 ny with the marked elongation of the body which we consider a departure from the norm 

 for primitive fishes. 



In discussing a series of elasmobranchs {Cladoselache, typically Devonian; Fleur acan- 

 thus, typically Permo'Carboniferous; Hyhodus, typically Jurassic; and Chlamydosela- 



